This invention relates generally to an electric power supplying technique, and more particularly to an AC power supply controlling method and device wherein the load current supplied from an AC power source to a load containing a substantial amount of inductance is controlled by means of electric valves.
Where an electric power is supplied from an AC power source to a load such as a welding machine having a lagging power factor, the load current is ordinarily controlled by a control system as shown in FIG. 1 of the accompanying drawings. In FIG. 1, a load current supplied from an AC power source 1 and flowing through a load containing a resistance 3 and an inductance 4 is controlled by electric valves 2, such as thyristors and the like, which are connected in parallel opposition. The ignition of the electric valves 2 is effectuated by an output gate signal V.sub.G delivered from a phase controlled 6. The phase controller 6 is operated by a synchronizing pulse signal picked up from the power source through a transformer 5. Within the phase controller 6, a capacitor 9 is charged through an adjustable (semi-fixed) resistor 7 and a variable resistor 8. When the charging voltage exceeds a predetermined value, a unijunction transistor 10 is turned on, thus energizing a pulse transformer 11. The pulse transformer 11 thus delivers from its secondary winding the aforementioned gate pulse V.sub.G which is applied to the electric valves. In the above described conventional arrangement, the resistor 7 has been manually adjusted so that the phase angle of the gate pulses V.sub.G for igniting the electric valves is always lagging from the angle of the power factor regardless of the control of the variable resistor 8. Once the resistor 7 is set as described above, the ordinary phase controlling operation of the load current can be achieved satisfactorily by adjusting the variable resistor 8.
However, where the power factor of the load is approximately zero as in a case where the load comprises a pure inductance, the control of the power supply becomes difficult. The reason will now be described with reference to FIGS. 2a and 2b.
In FIG. 2a, there are indicated a series of gate pulses V.sub.G, each igniting the electric valves 2 at an instant t.sub.1, and also the output voltage V.sub.O and the output current i flowing through the electric valves which are conducting throughout one-cycle of the power source voltage. As is apparent from the drawing, since a DC transient component always exists although not shown in the drawing, the conduction of the electric valves for the entire one-cycle period can be obtained only when the phase angles of the gate pulses V.sub.G for igniting the electric valves are made different from one cycle to other cycles.
In FIG. 2b, waveforms of the output voltage V.sub.O and the output current i are indicated for a case where the ignition phase angle of the electric valves is lagging from that of the power factor. In this case, since the DC component will be attenuated within about one half-cycle period, a symmetrical waveform of the current can be obtained by successive ignitions of the electric valves at a constant phase angle.
However, if the electric valve is ignited at a phase angle leading that of the power factor, the control of the electric valve tends to become discontinuous and uncontrollable because of the existence of the DC transient component. That is, there is always an angular range wherein the control becomes impossible.
For this reason, in the conventional control device as shown in FIG. 1, the adjustable resistor 7 has been manually readjusted each time the power factor varies so that the phase angles of the gate pulses will never lead the angle of the power factor regardless of the variation of the variable resistor 8 for the phase control of the load current.
In other words, in the case where a plurality of loads such as welding machines, the power factor of which widely varies depending on its load, are connected to the conventional power supply, it has been an ordinary practice that the ignition phase angle of the electric valve is manually limited to a value lagging from that of the power factor, thus limiting the use of the power supply in a reduced output condition. Such a procedure narrows the controllable range of the power supply apparatus, and the utilization factor of the same is thereby lowered.